The present invention relates to a method of measuring vehicle speed, suitable for inputting into a power steering apparatus for performing a power steering operation of a vehicle.
Current vehicles employ a power steering apparatus to facilitate a light, quick steering operation to reduce operator fatigue. More specifically, a pump using an engine as a power source, exerts an appropriate steering force to assist the operator in steering the vehicle. In such a power steering system, a steering angle, an engine speed, a vehicle speed, and the like are inputted into a controller (incorporating a microcomputer). A solenoid valve is driven in accordance with these parameters so as to control a hydraulic pressure supplied to a hydraulic pressure reaction chamber. This results in an optimal steering force.
In the power steering apparatus, vehicle speed measurement in the microcomputer is based on the number of pulses which are generated by vehicle speed signal inputs. The period of a generated pulse varies with vehicle speed. More specifically, as shown in FIG. 6, the number of pulses generated by inputs of a vehicle speed, A, during a predetermined sampling time interval, T, is counted. Vehicle speed measurement is based on this count value. The vehicle speed measurement value is updated after the next sampling time interval, T, has passed. Each time the predetermined sampling time interval, T, has passed, vehicle speed measurement is again performed, based on the number of generated pulses occurring during that time interval. Thus the vehicle speed measurement value used is updated after each time interval.
However, with the above-mentioned method of measuring vehicle speed, instantaneous disconnection of the power supply of the microcomputer, causes the microcomputer to be reset. As a result, the vehicle speed measurement value is left indefinite until the predetermined sampling time interval, T, has passed after the drive power supply has recovered from disconnection. In the power steering system, when the vehicle speed measurement value is left indefinite, the vehicle speed is set equal to zero. The resulting hydraulic pressure supplied to the hydraulic pressure reaction chamber increases the auxiliary steering force. This in turn reduces the actual steering force required of the operator.
The above scenario is especially dangerous when travelling at high speeds. At high speeds, the actual steering force required from the operator is high. However, should the power source be disconnected and the vehicle speed value be set to zero, the required actual steering force is now very low due to the increased auxiliary steering force. Such an abrupt change in actual steering force, unbeknownst to the operator, may result in incorrect steering with lethal consequences. The sampling time interval, T, must pass before the vehicle speed value is corrected. During this considerable amount of time, the dangerous condition exists.